Microwave electronics packages are frequently produced from aluminum alloys since such alloys have low weight and good thermal dissipation. These packages are generally machined from thick aluminum or an aluminum alloy block to house a microwave source. The housing has at least one wall with a through-opening over which a transparent waveguide window is hermetically sealed to allow microwaves to pass between the interior and exterior of the housing. It is necessary to have a reliable hermetic seal between the metal window frame and the metal electronics package housing.
Compatible metals may be interfaced with each other using standard procedures, such as laser welding, soldering, and the like. Dissimilar metals, e.g., metals characterized by differing thermal expansion properties, melting points, weld incompatibilities and the like, do not reliably interface using such standard laser welding and soldering procedures. For example, iron cannot be physically laser welded to aluminum, and solder joints between iron and aluminum have a definite thermal fatigue cycle life. As a result, an iron-based metal waveguide window frame cannot be reliably soldered or laser welded to an aluminum electronics package and undergo sustained periods of reliable operation.
Currently, waveguide windows are mounted to electronics packages using complicated solder techniques. A glass window is sealed to a metal frame using a conventional glass-to-metal sealing technique. The metal frame is typically made of an iron-based alloy such as KOVAR.RTM.. The metal frame is designed to interface with the glass in a standard fashion. The glass window/metal frame sub-assembly is mounted and soldered to a copper bellows. The copper bellows is soldered to the aluminum electronics package. The copper bellows helps relieve the thermal stresses caused by the different thermal expansion properties of the metal frame and the aluminum housing. The joint between the window assembly and the package housing must remain hermetic throughout repeated thermal cycling to provide reliable operation.
The use of the above-mentioned copper bellows between the glass window/metal frame subassembly and the electronics package housing has a number of drawbacks. It is difficult to coordinate soldering the window-to-bellows and the bellows-to-housing at the same time. Additionally, the window assembly is mounted within a small recess in the package housing which forms an abutment face to which the copper bellows is soldered. Since the copper bellows must be sealed to the package housing within the recess, it is difficult to obtain an accurate and reliable solder joint within the recess. Furthermore, the temperature required to solder, which is typically about 200.degree. C., often stresses the glass window and breaks it.
Soldering techniques typically produce a rather high heat affected zone (HAZ), which may extend into the glass-to-metal seal (GTMS) zone in waveguide assemblies. Use of joining techniques producing a large HAZ may adversely affect the integrity of the glass window, as well as the integrity of the connection between the glass window and the metal frame. To maintain the integrity of the GTMS zone, it is preferable to use joining techniques having a low or narrow HAZ. It is known that laser welding produces a relatively low HAZ. However, it is not possible to hermetically join metals having dissimilar thermal properties, such as copper or iron alloys with aluminum or an aluminum alloy.
Methods are known for interfacing dissimilar metals. For example, an aluminum body and a standard iron-based body may be joined through the use of a transition bushing. A transition bushing is a metal-to-metal interface bushing fabricated from a dissimilar metal sheet. A dissimilar metal sheet is a sheet of metal consisting of two or more layers of dissimilar metal which have been joined together by, for example, an explosive weld. An explosive weld connotes the metallurgical bond created at the point of impact when one metal is driven against another by the force of an explosion.
U.S. Pat. No. 5,001,299 discloses an explosively bonded electronics package having an aluminum housing with upper and lower layers of a dissimilar metal clad thereto. Holes are drilled in the side of the housing to accommodate sealed wires that are connected to a ceramic substrate attached to the floor of the housing.
U.S. Pat. No. 5,109,594 discloses a method of making a sealed transition joint for an electrical contact pin connection unit. In this device, the pin is sealed to the connector by a glass seal having a uniform shape. In order to increase reliability of the glass seal, it must be relatively thick. The transition joint is made of dissimilar metal layers where one layer is made of the same material as the package housing and the other layer is made of the same material as the pin connector.
U.S. Pat. No. 5,110,307 discloses a bi-metal flange providing a hermetic seal between a pin connector and an electronics package housing.
U.S. Pat. No. 5,298,683 discloses a connector formed from at least two dissimilar metals that is capable of sealing a hermetic feed-through in an electronics package.
These prior art devices do not disclose a waveguide window assembly having a metal window frame for mounting a thin glass window that is capable of being hermetically sealed to an electronics package, preferably by a laser weld, without adversely affecting the integrity of the sealed glass window.
U.S. Pat. No. 5,430,257 discloses a waveguide window assembly in which a buffer section, composed of materials which progressively vary the coefficient of thermal expansion, is mounted between the housing and the window frame. A preferred embodiment employs a buffer section composed of two materials arranged in side-by-side relationship, with a deep groove formed near the interface between the two materials to provide stress relief. Soldering is the preferred method for joining the window to the frame and the frame to the housing. High thermal stresses are generated, however, during soldering, which result in high breakage rates for the delicate transparent window. The assembly disclosed in this patent furthermore suffers performance problems as a consequence of discontinuities in the microwave ground path along the waveguide tube where it joins the waveguide window assembly.
The waveguide assembly of the present invention is thus capable of being hermetically sealed in an electronics package in such a way that installation of the waveguide window in the electronics package and repeated thermal cycling of the assembly do not result in any breakage or deterioration in the performance properties. The assembly of the present invention also provides for a continuous microwave signal ground path, thereby improving signal transmission and fidelity.